The present invention relates to a nozzle-shaped structure or breaking chamber for self-blasting compressed gas electric circuit breakers which provide considerably improved performance over prior circuit breakers of the same type.
Self-blasting compressed gas electric circuit breakers are well known and widely employed in electric power generating plants. Axial blast breaking elements for such breakers are particularly well known and a substantial amount of engineering effort has been devoted to the modification and improvement of the nozzle structure (or breaking chamber) of these breakers with the objective of obtaining improved performance. For example, compressed gas circuit breakers having decompression side holes and ring-like grooves in the end zone of the nozzle structure have been developed previously. In these breakers, self-blast of the quenching gas is produced by action of a fixed piston in co-operation with a nozzle structure that is integral with the circuit breaker movable contact. In particular, in circuit breakers of this type, the ratio between the sum of the areas of the smallest sections of the decompression side holes and the area of the smallest flow section (or neck) through the nozzle is equal to 0.5
U.S. Pat. Nos. 3,668,352 granted June 6, 1972, 3,670,125 granted June 13, 1972 and pending U.S. patent application Ser. No. 275,219 filed July 26, 1972, all by the present inventor, disclose nozzle structures for axial self-blasting breaking elements for compressed gas electric circuit breakers which are provided with decompression side holes and ring-like grooves. These holes and grooves are shaped to provide a significant improvement over prior art structures, the decomposition gases developed from the materials making up the nozzle and the quenching gas being caused to flow in a more regular manner. This results in the deionization of the insulating medium and dielectric strength recovery being substantially improved.
The design of the downstream (or end) zone of nozzles of the type disclosed in the previously mentioned U.S. Patent Nos. and patent application is dependent on the rated service voltage of the circuit-breaker according to the formula ##EQU1## where L is the length of the nozzle end zone, in mm, and U.sub.n is the circuit-breaker service rated voltage, in kV.
This experimental relationship can be started in the following form: ##EQU2## where 1.5 (.sqroot.2/.sqroot.3) U.sub.n, in kV represents the effective value of the recovery voltage occurring at the ends of that pair of contacts which first interrupts the current.
It has been found, however, that existing breakers do not have optimum efficiency, particularly with regard to the outflow of the arc quenching gas as well as the outflow of the decomposition gases developed from the materials making up the nozzle. Further, rapid and effective deionization of both the electric arc plasma and the surrounding space (that is, the inner part of the nozzle where the movable contact separates from the fixed contact) have not been attained in axial blast breaking elements provided with nozzle structures developed heretofore.